Air conditioners are known where a plurality of indoor units are connected to at least one outdoor unit by a liquid pipe and a gas pipe. Among such air conditioners, an air conditioner has been proposed where sufficient air conditioning ability can be displayed at each indoor unit by controlling a refrigerant circuit in consideration of the difference in height between the installation place of the outdoor unit and the installation places of the indoor units and the difference in height between the indoor units.
For example, in an air conditioner described in JP-A-4-28970, an outdoor unit provided with a compressor, a four-way valve, an outdoor heat exchanger, an outdoor fan and an outdoor expansion valve is installed on the ground, whereas two indoor units each provided with an indoor heat exchanger, an indoor expansion valve and an indoor fan are installed with a difference in height therebetween in higher places than the outdoor unit (JP-A-4-28970, one indoor unit is installed on the first floor of a building and the other indoor unit, on the fourth floor in higher places than the outdoor unit), and the two indoor units and the outdoor unit are connected by refrigerant pipes to form a refrigerant circuit.
When cooling operation is performed by this air conditioner, since the liquid refrigerant condensed at the outdoor unit and having flown from the outdoor unit into the liquid pipe flows to each indoor unit against gravity, the pressure of the liquid refrigerant on the upstream side (the outdoor unit side) of the indoor expansion valve of the indoor unit installed in the higher position is lower than the pressure of the liquid refrigerant on the upstream side of the indoor expansion valve of the indoor unit installed in the lower position.
For this reason, the difference between the refrigerant pressure on the upstream side of the indoor expansion valve of the indoor unit installed in the higher position and the refrigerant pressure on the downstream side thereof (the indoor heat exchanger side) is small compared with the difference between the refrigerant pressure on the upstream side of the indoor expansion valve of the indoor unit installed in the lower position and the refrigerant pressure on the downstream side thereof. Since the amount of refrigerant flowing through the indoor expansion valve decreases as the difference in pressure between on the upstream side and on the downstream side of the indoor expansion valve decreases, a large amount of refrigerant flows in the indoor unit installed in the lower position, whereas the amount of refrigerant flowing in the indoor unit installed in the higher position decreases and there is a possibility that sufficient cooling ability is not obtained.
Therefore, in the air conditioner disclosed in JP-A-4-28970, the degree of opening of the indoor expansion valve of the indoor unit installed in the lower position is made smaller by a predetermined degree than the degree of opening of the indoor expansion valve of the indoor unit installed in the higher position, whereby the amount of flow of the refrigerant in the indoor unit installed in the lower position is decreased and the amount of flow of the refrigerant in the indoor unit installed in the higher position is increased. Thereby, even in the air conditioner where the outdoor unit is installed on the ground and the two indoor units are installed with a difference in height therebetween in higher places than the outdoor unit, sufficient cooling ability can be displayed by the indoor unit installed in the higher position. When heating operation is performed by an air conditioner where indoor units are installed with a difference in height therebetween and an outdoor unit is installed in a higher position than the indoor units unlike the air conditioner of JP-A-4-28970, a problem described below arises.
In heating operation, while the gas refrigerant discharged from the compressor flows into the indoor heat exchanger of each indoor unit to be condensed, since the liquid refrigerant condensed at the indoor heat exchanger and having flown into the liquid pipe flows against gravity toward the outdoor unit installed in the higher position, the lower the position in which an indoor unit is installed is, the more difficult it is for the liquid refrigerant having flown from the indoor unit into the liquid pipe to flow toward the outdoor unit. Thereby, the pressure of the liquid refrigerant on the downstream side (the outdoor unit side) of the indoor expansion valve of the indoor unit installed in the lower position becomes higher than the pressure of the liquid refrigerant on the downstream side of the indoor expansion valve of the indoor unit installed in the higher position. Consequently, the difference between the refrigerant pressure on the upstream side (the indoor heat exchanger side) of the indoor expansion valve of the indoor unit installed in the lower position and the refrigerant pressure on the downstream side thereof becomes small compared with the difference between the refrigerant pressure on the upstream side of the indoor expansion valve of the indoor unit installed in the higher position and the refrigerant pressure on the downstream side thereof.
Since the amount of refrigerant flowing through the indoor expansion valve decreases as the difference between the refrigerant pressure on the upstream side of the indoor expansion valve and the refrigerant pressure on the downstream side thereof decreases, a large amount of refrigerant flows in the indoor unit installed in the higher position, whereas the amount of refrigerant flowing in the indoor unit installed in the lower position decreases and there is a possibility that sufficient heating ability is not obtained in the indoor unit. Therefore, it is considered to perform control based on a principle similar to that of the air conditioner of Patent Document 1 so that the degree of opening of the indoor expansion vale of the indoor unit installed in the lower position is always higher than the degree of opening of the indoor expansion valve of the indoor unit installed in the higher position. Thereby, the amount of refrigerant flowing in the indoor unit installed in the lower position becomes large compared with the amount of refrigerant flowing in the indoor unit installed in the higher position, so that the heating ability at the indoor unit installed in the lower position can be improved.
Since it becomes more difficult for the liquid refrigerant having flown from the indoor unit installed in the lower position to flow in the liquid pipe toward the outdoor unit as the difference in height between the indoor unit installed in the lower position and the indoor unit installed in the higher position increases, the difference in pressure between the liquid refrigerants on the downstream side of the indoor expansion valves of these increases, and the difference between the refrigerant pressure on the upstream side of the indoor expansion vale of the indoor unit installed in the lower position and the refrigerant pressure on the downstream side thereof decreases. For this reason, it is necessary that the degree of opening of the indoor expansion valve of the indoor unit installed in the lower position be a degree of opening corresponding to the difference in height between the indoor unit installed in the lower position and the indoor unit installed in the higher position. That is, it is necessary that the degree of opening of the indoor expansion valve of the indoor unit installed in the lower position be increased as the difference in height between the indoor unit installed in the lower position and the indoor unit installed in the higher position increases.
However, the difference in height between the indoor unit installed in the lower position and the indoor unit installed in the higher position is large, and the liquid refrigerant having flown from the indoor unit installed in the lower position into the liquid pipe does not flow toward the outdoor unit; that is, when the liquid refrigerant stays below the liquid pipe, even if the degree of opening of the indoor expansion valve of the indoor unit installed in the lower position is made full opening, no refrigerant flows in the indoor unit and no heating ability is displayed (heating cannot be performed).